1. Field of the Invention
The present invention relates generally to optical waveguides for the transmission of electromagnetic energy. The present invention relates more particularly to optical fibers suitable for use with high optical energies, and to devices using them.
2. Technical Background
Fiber lasers have many attractive properties that make them suitable for various industrial applications. Such properties can include one or more of good beam quality, easy thermal management, compact size, and good efficiency. Fiber lasers are therefore often preferred to conventional types of lasers, such as solid-state and gas lasers. Fiber lasers are able to produce optical output in the several kW range with excellent beam quality. Thus, these lasers can be used for macro-machining applications like welding and cutting of metal. Furthermore, fiber lasers lend themselves to operation with ultra-short pulses by a method of mode-locking, enabling them to be used in micro-machining applications as well.
As any laser, a fiber laser can include a gain medium, an optical resonator, means of coupling energy into the gain medium, and means of extracting light out of the optical resonator. The gain medium in a fiber laser can include a length of an optical fiber, the “active fiber,” which is coupled to a source of pump energy. Typically the core of the active fiber is doped with optically active atoms such as rare earth atoms (e.g., Er or Yb). The optical resonator can be formed by surrounding the gain medium with mirrors that, when properly aligned with respect to the active fiber, force some of the light emitted by the active atoms to bounce between the mirrors through the gain medium and get amplified. The mirrors can be either bulk optical mirrors, or they can be directly fabricated into optical fibers. In the latter case they are usually fiber Bragg gratings (FBGs), but other fiber-based or free space mirrors can also be used. Fiber-based mirrors are attractive since they can be directly attached or spliced to other fibers with very low optical losses. The mirrors, or typically only one of the two mirrors, are made only partially reflective to provide a route for extraction of light out of the optical resonator. In fiber lasers, the extracted light can be further guided with a length of optical fiber close to the point of interest, such as a work-piece. The extracted light thus forms a beam of laser light that can be used in the final application.
Fiber amplifiers are likewise attractive devices that are suitable for a variety of applications. In a fiber amplifier, an optical signal to be amplified is transmitted through the active fiber; pump energy stimulates the amplification of the optical signal in the active fiber.
In both types of active fiber devices, the power levels in the active fiber are often in the range of tens of watts, even up to over a kilowatt. Such high powers can result in deleterious nonlinear effects in the fiber itself, such as self-phase modulation, Raman scattering and Brillouin scattering. Accordingly, active fibers with large mode areas are often used. When the power is spread over a larger area, the average power density is lowered, thereby reducing the magnitude of these nonlinear effects. However, in order to retain single mode operation, the numerical aperture of the active fiber must be reduced, thereby increasing the bend sensitivity of the fiber. Such fibers can complicate use in actual devices, as they cannot be coiled in a space-efficient manner.
Accordingly, there remains a need for optical fibers that provide effectively single mode operation with low loss, even when in a coiled configuration.